The patrocinial [system of patrons and clients superimposing their rule on the Roman Republican system had expanded into the imperial principate and] was the institution that made the new ruler the existential representative for the vast agglomeration of conquered territories and peoples. Obviously, the instrument was brittle. Its effectiveness depended on the experience of the patrocinial relation as a sacramental bond in the Roman sense.
The new Augustus saw the problem; and his legislation for moral and religious reform must be understood, at least in part, as the attempt of reinforcing sacramental sentiments that had been waning even among the Romans at the time of Varro's Antiquities.
In face of the vast oriental population the task was hopeless, especially since the Easterners streamed into Rome in ever increasing numbers and were clinging to their non-Roman cults in spite of all prohibitions; and the task became still more hopeless when the emperors themselves ceased to be Romans, when the Julian dynasty was followed by the provincial Flavians, by the Spaniards, the Syrians, and the Illyrians.
The remedy for the sacramental deficiency in the position of the emperor was found only gradually, on a tortuous path of experimentation and failure. The divinization of the emperor, following the model of Hellenistic kingship, proved insufficient. It also had to be determined which divine power he represented among the mass of cult divinities in the empire.
Under the pressure of this problem the religious culture of the Roman Mediterranean underwent a process that usually is called syncretism, or theokrasia, mixture of the gods. The evolution is not singular; it is in substance the same process that the Near Eastern empires had undergone at an earlier time, the process of reinterpreting the multitude of local cult divinities in the politically unified area as the aspects of one highest god who then became the empire-god.
Under the peculiar conditions of the civilizationally mixed Roman area, experimentation with such a highest god was not easy. On the one hand, the god could not be a conceptual abstraction but had to have an intelligible relationship to one or more concretely experienced gods who were known as high; on the other hand, if the relationship to a concretely existing god became too close, his value as a god above all known special gods was in danger.
The attempt of Elagabalus (218-222) to introduce the Baal of Emesa as the highest god to Rome miscarried. A circumcised Caesar who married a Vestal virgin in order to symbolize the union between Baal and Tanit proved too much of a strain on the Roman tradition. He was murdered by his praetorian guards.
The Illyrian Aurelian (270-275) tried with better success when he declared a sufficiently nondescript sun god, the Sol Invictus, as the highest god of the empire and himself as his descendant and representative. With some variation under Diocletian (284-305) the system lasted until A.D. 313.
The fact that the empire cult was a subject of experimentation should not deceive us, however, about the religious seriousness with which these experiments were undertaken. Spiritually the late Roman summodeism had approached closely enough to Christianity to make conversion almost a slight transition. There is extant the prayer of Licinius before his battle against Maximinus Daza in 313. An angel appeared to Licinius in the night and assured him of victory if he and the army would pray it:
Highest God, we pray to thee,
Holy God, we pray to thee.
All justice we command to thee,
Our weal we command to thee,
Our realm we command to thee.
By thee we live, by thee we are victorious and successful.
Highest, Holy God, hear our prayers.
We raise our arms to thee,
Hear us, oh Holy, Highest God.
Story and prayer are reported by Lactantius, with the understanding that the victory was due to a conversion similar to Constantine's in the year before. The Christianity of Licinius is at least doubtful in view of his anti-Christian policy in subsequent years, but the prayer, which could as well have been prayed by his pagan opponent Maximinus, appeared as a confession of Christianity to Lactantius.